As a chassis engineer, I can triple confirm that both M and non-M struts are twin tubes. The non-M pictured has a rebound spring, which is an internal spring that acts against the normal coil spring to increase roll stiffness. In the resting state, the spring is extended, which holds the rod in the position you see in the picture. If you grab the rod and pull really hard, you would compress the spring and you'd be able to pull the rod out almost as far as the M3 strut.

Rebound springs act sort of like a stiffer stabilizer bar, with some key differences:

They also reduce body float and prevent the suspension from topping when you hit a large pothole, which helps prevent wheel and tire damage

okay guys i have a question... I lowered my car(330i) yesterday on tein-s tech springs. after installing the tein springs, i found out that all 4 corners shocks/struts were blown. My car was sitting really low (about 2 inch drop). The mechanic said that my car was sitting really low because my shocks/springs were out. I can't afford koni/bilsteins shocks atm and there is someone local to me selling a full set of m3 suspension. From research, i found out i can directly swap out the rear shocks of an m3 to non-m but how would i go about doing the front. Would i still be able to use my tein (non-m) springs on m3 front struts and top hats? What else would i need to replace and/or tweak. any info would help. thanks guys!

okay guys i have a question... I lowered my car(330i) yesterday on tein-s tech springs. after installing the tein springs, i found out that all 4 corners shocks/struts were blown. My car was sitting really low (about 2 inch drop). The mechanic said that my car was sitting really low because my shocks/springs were out. I can't afford koni/bilsteins shocks atm and there is someone local to me selling a full set of m3 suspension. From research, i found out i can directly swap out the rear shocks of an m3 to non-m but how would i go about doing the front. Would i still be able to use my tein (non-m) springs on m3 front struts and top hats? What else would i need to replace and/or tweak. any info would help. thanks guys!

1) I've already addressed the shocks/struts being blown. I've told the forum over and over that your stock struts/shocks, if original OE, are blown after 40k miles.

2) The used M3 stuff you're going to buy will also be considerably reduced in capability if not severely blown.

My advice to you is to save up for stuff you really want instead of installing junk on your car then spending even more money and time later installing the stuff you really want.

Don't nickel and dime things. Do it right the first time. GL!

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mango...what is the benefit or installing m3 suspension on a non m3 car? does the cost justify the experience?

IMO, none. Installing an M3 "suspension" usually means M3 struts and shocks. M3 struts and shocks are usually used (who buys brand new OE M3 struts and shocks) and used stock BMW struts/shocks are usually degraded or blown after 40k. So by installing used worn suspension parts on your car, you aren't really doing yourself any favors.

If one wants to go through the trouble, then sure, no problem. But installing used M3 stuff is more trouble than its worth, IMO, and may end up causing your car to handle worse or best case, no better.

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IMO, none. Installing an M3 "suspension" usually means M3 struts and shocks. M3 struts and shocks are usually used (who buys brand new OE M3 struts and shocks) and used stock BMW struts/shocks are usually degraded or blown after 40k. So by installing used worn suspension parts on your car, you aren't really doing yourself any favors.

If one wants to go through the trouble, then sure, no problem. But installing used M3 stuff is more trouble than its worth, IMO, and may end up causing your car to handle worse or best case, no better.

The M3 suspension have benefits if you want a sportier ride and more aggressive alignment compared to non-M suspension, and the design of the strut mounts is stronger and longer lasting.

If you don't want to make your car sportier and deal with a rougher ride and altered alignment settings, then no, there aren't any benefits. Functionally, everything is very similar, that's why they can be swapped.

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that has to be the worst thing you can do to a set of springs. cutting a section of coil out that's not a "dead section" is like changing the whole spring rate and matched set of springs. that's Honda stuff. Don't cut your coils in your springs.

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Switching gears a little bit, how would a set of Non-M ST Coilovers fit on an M3?

It seems like the rear shock/spring combo is a direct fit.
Aside from using M3 front strut mounts, would anything else be needed for the front installation?

Yes, the rear setup is a direct fit.
M3 strut mounts don't fit properly on non-M dampers. The fronts will fit with non-M strut mounts, but keep in mind that the camber and caster will change relative to the original M3 suspension. Camber plates would be a good addition.

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Yes, the rear setup is a direct fit.
M3 strut mounts don't fit properly on non-M dampers. The fronts will fit with non-M strut mounts, but keep in mind that the camber and caster will change relative to the original M3 suspension. Camber plates would be a good addition.

anyone's guess. the more you drive on a misaligned car the faster your tires will wear. it might introduce dangerous handling situations at the limit too. best to do it the right way all at once.

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Yes, the rear setup is a direct fit.
M3 strut mounts don't fit properly on non-M dampers. The fronts will fit with non-M strut mounts, but keep in mind that the camber and caster will change relative to the original M3 suspension. Camber plates would be a good addition.

Thanks Kalim.

So aside from the offset camber, the non-M coilovers with non-M front mounts would be a direct bolt on to an M3?

You'll have slightly less caster and more negative camber than OEM M3 suspension at the same ride height, which should be fine. The toe will need to be corrected soon, though. I don't think I'd go as far as driving for a few months with the front end misaligned.

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So aside from the offset camber, the non-M coilovers with non-M front mounts would be a direct bolt on to an M3?

You're welcome.

Yes, they should drop right in. You will need to remove the alignment pin on the passenger side non-M mount to fit an M3 strut tower, IIRC. Looking from the front, the M3 strut mount pins are both behind the stud whereas the non-M mounts are universal left/right, so one pin ends up being in front of the stud.

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You'll have slightly less caster and more negative camber than OEM M3 suspension at the same ride height, which should be fine. The toe will need to be corrected soon, though. I don't think I'd go as far as driving for a few months with the front end misaligned.

I was going to get it realigned,I just didn't know if the geometry would be off to bad that I couldn't